U.S. Pat. No. 4,194,505 discloses an injection device used primarily for injecting insulin. It has a mechanism for effecting an injection process that is movable counter to the force of a spring into a cocked position, out of which it can be released to act upon the piston of a cartridge containing insulin. An adjusting element on the housing of this known injection device is used as a stop for the aforementioned mechanism and makes it possible to vary the effective length of the mechanism and hence the injection dose, within certain limits. A scale with which the adjusting element can be set to a desired value is provided on the housing. The known device is embodied to hold special cartridges, which contain a certain quantity of insulin, such as 20 units. If the patient needs only 17 units, he can make a setting on the adjusting element accordingly. To do so, he first sets the maximum dose, which is equivalent to a zero line on the scale of the device. Beginning there, he adjusts by three lines on the scale, corresponding to the three units that he does not want to inject, using the adjusting element. This adjusts the stop on the housing for the movable mechanism. Next, the patient initiates the injection, whereupon the needle is first inserted into the patient and then the desired dose of insulin is injected from the cartridge. This device is complicated to use: Cartridges of various doses are needed for it, namely cartridges containing 10, 20, 30, 40, 50 and 60 units of insulin. If the patient is blind, he can easily mistake such cartridges for one another, and the danger then exists that he may inject too much insulin. The calculations that must be made in using the device are not exactly simple either, and may be too difficult for some patients. In so-called intensified insulin therapy, the patient must inject a different amount of insulin after each meal, and mistakes in calculation are easy to make; or the patient may simply reuse the old setting for the device. If the patient needs 8 units of insulin after breakfast, 15 units after lunch and 12 units after dinner, for instance, then with this known device he has to use one 10-unit cartridge for breakfast and set the device to minus two units. For lunch and dinner, he has to use a 20-unit cartridge, setting the device to minus five units at noon and to minus eight units in the evening. This all requires great care and may easily lead to mistakes.
A so-called rumen injector is also known from German Patent 3 527 066. In this device, the piston rod of the injection piston has a handle on its free end, and an adjustable stop provided on the device facing the handle limits the motion of the handle and thereby determines the injection dose. With this device, the veterinarian inserts the hypodermic needle manually into the animal, and the insertion initiates an automatic injection process, in which a spring presses the piston rod so far into the injection cylinder that the handle strikes the aforementioned stop. The disadvantage of this device is that the injection cylinder has to be reloaded before each injection, which does not allow the use of cartridges or other containers holding more than one dose of the fluid to be injected.
From European Disclosure Document Al 0 058 536, an injection device is known with which a patient can inject more than one dose of insulin from one cartridge. To do so, he adjusts the desired insulin dose by turning an adjusting wheel, then inserts the needle into himself, and then turns the pre-adjusting wheel back to its zero position. This screws a threaded part out of a sleeve, presses it against the piston in the cartridge, and forces insulin out of the cartridge into the patient through the needle. However, this has the disadvantage that the device can be used only on the front part of the body, while with so-called intensified insulin therapy one is forced in practice to make injections on the back part of the body as well, to avoid hematomas and cornification from overly frequent injection at the same site. Also, injection by manually turning a pre-adjusting wheel is very painful for some patients.
Such injection devices are complicated devices. The patient must therefore be carefully instructed, usually in a hospital, in their use. In addition, the use of such devices should be made as simple and understandable as possible, so that the patient will not find the device burdensome to use and will continue his therapy regularly and without making mistakes.